Control for frequency converters

ABSTRACT

Control means for an AC/AC frequency converter for induction heating employing solid state switching devices of the type shown in application for U.S. Letters Patent Ser. No. 737,639 filed June 17, 1968, (U.S. Pat. No. 3,546,562, granted Dec. 8, 1970), which operates essentially at the resonant frequency of the tuned load circuit, comprising circuits individually sensing the high frequency output voltage and current, the low frequency input voltage and combining said individual signals to automatically adjust firing signals delivered to the solid state switching devices to minimize transients.

United States Patent Havas et al.

1541 CONTROL FOR FREQUENCY CONVERTERS [72] Inventors: George Havas,Youngstown, Ohio; Shashi Bhushan Dewan, Toronto,

Canada [73] Assignee: Ajax Magnethermic Corporation,

. Warren, Ohio [22] Filed: Jan. 4, 1971 [21] Appl. No.: 103,569

[52] US. Cl. ..321/7, 321/11, 321/19,

321/40, 321/52, 321/69 R [51] Int. Cl. ..H02m 5/30 [58] Field ofSearch..32l/3, 4, 7, 11,19, 40, 52, 321/58, 60, 61, 69

[56] References Cited UNITED STATES PATENTS 3,568,033 3/1971 Gyugyi..321 7 3,517,300 6/1970 McMurray ..321/69 R 1451 Aug. 22, 19723,424,970 l/1969 Ross ..321/18 X 3,376,487 4/1968 Bixby ("321/19 X3,461,374 8/1969 Rhyne, Jr ..32l/l9 X 3,566,245 2/ 1971 Blokker et a1...321/19 X Primary Examiner- WiIliarn M. Shoop, Jr. AttorneyJ. H. Slough57 ABSTRACT Control means, for an AC/AC' frequency converter forinduction heating employing solid 1 state switching devices of the typeshown in application for US. Letters Patent Ser. NO. 737,639 filed June17, 1968, (US. Pat. No. 3,546,562, granted Dec. 8, 1970), which operatesessentially at the resonant frequency of the tuned load circuit,comprising circuits individually sensing the high frequency outputvoltage and current, the low frequency input voltage and combining saidindividualsignals to automatically adjust firing signals delivered tothe solid state switching devices to minimize transients.

24 Clains, 13 Drawing Figures CURRENT LOGIC 42 COMPA- RATOR LOGICVOLTAGE LOGIC -4/- ENVELOPE LOGIC Patented Aug. 22, 1972 3,686,558

8 Sheets-Sheet 1' GfORCE HA l/AS SHASH/ BHUSHA DEW/1N Fl ICC vlNl/ENTORS ATTORNEY Patented Aug. 22, 1972 3,686,558

8 Sheets-Sheet 5 Fif 3 AAAAVO VV Fi a; 4 //WE/\/ TORS GEORGE HAVASSHASH/ BHUSHAN DEW/1N J. H. SLOUGH ATTORNEY Patented Aug. 22,1972 I 8Sheets-Sheet 4 fi r s CURRENT POWER 0 SHIFT O SHIFT F t 6a VOLTA G E OSHIFT ic 6c Patented Aug. 22, 1972 8 Sheets-Sheet 6 +THYR ISTORS-THYRISTORS Io LOGICOUTPUT PU LS E5 TO +THYRISTORS 'FTILs ES TO -THYRISTORS Patented Aug. 22, 1972 8 Sheets-Sheet 6 +THYRISTORS Io LOGICOUTPUT PULS E5 TO +THYRI5TORS THYRISTORS PULSES TO "THYRISTORS INVENTORSGEORGE HA VAS SHASH/ BHUSHAN DEW/4N L]. H, \SLOUGH ATTORNEY PatentedAug. 22, 1972 3,686,558

8 Sheets-Sheet 8 A E5 c I (A) A A /Nl/EN7'OR-5 GEORGE HAVAS SHA-SH/BHUJHAN DEW/1N J. H. SLOUGH A 7'7'OR/VEY CONTROL FOR FREQUENCYCONVERTERS In the control means of our invention for a frequencyconverter of the type referred to herein,'converting three-phasealternating current power into single-phase higher variable frequencyalternating current power that is particularly adapted for use ininduction heating and melting, which converters operate essentially atthe resonant frequency of a tuned load circuit, comprising circuitindividually sensing the high frequency output voltage and current, thelow frequency input voltage and in which individual signals are combinedto control firing of solid state switching devices, a fixed intervalbetween the pulses which cause the solid state switching devices tobecome conductive and the time at which the output voltage reaches thezero cross-over instant is maintained when the converter is operatingunder normal steady state conditions. Under various transientconditions, however, such as the initiation of the operation of theconverter, sudden load changes, changes in the intended power level,line voltage surges, etc., the pattern of instantaneous output voltageand current will depart substantially for a period of a few half-cyclesfrom the normal stead state operating pattern. If the fixed timerelationship described above is maintained during these transientperiods, the free running oscillation of the circuit may be disturbed,the operation may be interrupted non-destructively, or the solid stateswitching devices may even be destroyed. The improved control means ofour invention prevents such disturbance or interruption of the circuitor damage to the switches.

An object of this invention is to provide improved control means for aconverter of the type set forth. It is also an object of this inventionto provide improved control means in a converter of the type referred toto accommodate the transient changes by an automatic adjustment in thetiming of the firing pulses.

It is a further object of this invention to provide control meanswhereby initiation of the operation of the converter is achieved with aminimum of deleterious transient effect.

A still further object of the invention is to provide an improved methodfor power'control in a converter of the type referred to wherebytransients are minimized.

Another object of the invention is to provide improved control means fora converter of the type referred to whereby damage to the solid stateswitching devices is avoided.

A still further object of the invention is to provide improved controlmeans for a converter of the type referred to whereby continued andsmooth safe operation of the converter is achieved.

Another object of the invention is to achieve a converter of the typereferred to with improved operating characteristics and efficiency.

Other objects of our invention and the invention itself will become morereadily apparent by reference to the accompanying drawings and appendedspecification, in which drawings, like parts are designated-by likereference characters.

FIG. 1 shows a frequency converter circuit connected between athree-phase low frequency source of alternating current and a singlephase high frequency load,

FIG. la is an alternate method of load connection employing an isolationtransformer;

FIG. 2 shows the frequency converter circuit of FIG. 1 with thethyristor control system of our invention applied thereto;

FIG. 3 illustrates the relationship between the threephase line toneutral voltage input of the converter circuit and the state of theenvelope logic at maximum power;

FIG. 4 illustrates the relationship between the high frequency circuitoutput voltage and the voltage logic of the control of the invention;

FIG. 5 illustrates the relationship between output current and thecurrent logic of the control of the invention;

FIG. 6a, 6b and 6c is a diagrammatic sketch showing the effect of theenvelope phase shift on output power, output current, and outputvoltage, respectively;

FIG. 7a illustrates the relationship between the single-phase outputvoltage and current of the converter and timing of the gating pulses ofthe positive and negative thyristors;

FIG. 7b illustrates the relationship between the single-phase outputvoltage and positive output current of extended duration and the timingof the output gating pulses with relation thereto;

FIG. 8 is a diagrammatic sketch showing the time relationship betweenthe line to neutral input voltage, the envelope, the total outputcurrent, and the current through-a selected pair of thyristors l0 and 11for maximum output. voltage; and

FIG. 9 is a diagrammatic sketch showing the time relationship betweenthe line to neutral input voltage, the envelope, the output current, andthe current through the selected pair of thyristors for minimum outputvoltage.

In FIG. 1 a converter of the type described, set forth and claimed inco-pending application filed June 17, 1968, for Frequency Converter,Ser. No. 737,639 (US. Pat. No. 3,546,562, granted Dec. 8, 1970) of thepresent inventors, is illustrated. In such converter, anodes l3, l4, 15of switches 10, ll, 12 are connected each to a phase of the source A, B,and C and can be termed positive conducting switches. The remainingthree switches 16, 17, 18 have their cathodes 19, 20, 21 connected tothe said three phases of the source and are termed negative conductingswitches. The cathodes 25, 26, 27 of the positive switches 10, ll, 12each having a common connection and are connected to an end terminal 22of a center tapped inductance winding 24. The anodes 28, 29, 30 of thenegative switches 16, 17, 18 have a common connection and are connectedto an end terminal 23 of the inductance winding 24. The center tappedterminal 31 of the inductance winding 24 is connected to a highfrequency terminal 32 of the load. The converter also includes threecapacitors 34, 35, 36 connected in wye, with one terminal connected toeach of the phases of the source A, B, and C and a second terminal ofeach connected to a common point connected to a second high frequencyoutput terminal 33 of the load.

The load connected to the output terminals 32, 33 is designated by L andis represented by an inductance coil C and resistance R in seriestherewith connected in parallel with capacitance 37 which tunes thecombination of the capacitance and the load to a higher value frequencythan the polyphase lower frequency supplied. Alternatively, the load canbe connected to the output terminals by means of an isolationtransformer 38 as shown in FIGS. 1a and 2. In operation, in the circuitof FIG. 1, one of the positive switches 10, 1 1, 12 is caused to conductand current to flow through the selected switch and then through aportion of the inductance 24, through the load, and then through thecapacitance and back to the source. As the capacitances conduct the loadcurrent they are charged so that the voltage across said switch is firstreduced to zero, then to a negative value, and current ceases to fiow tothe load.

One of the negative switches is then caused to conduct and currentstarts to flow in the series resonant circuit in an opposite directionfrom that described above, through the capacitance, through the load Land through the remaining portions of the inductance 24. Currentcontinues to flow through said portion of the inductance until thecapacitance is charged sufficiently to cause the voltage across theselected negative switch to be reduced to zero and then to a negativevalue and load current then ceases. It will thus be seen that theoperation of the converter generally involves alternate operation ofpositive conducting switches and negative conducting switches and onlyone polarity is conducting at any time.

In the converter of our invention, as shown in FIG. 2 we haveincorporated a control means in the circuit of FIG. 1 which consists ofa so-called envelope logic 40 which derives signals from the voltage ofthe separate phases A, B, C; voltage logic 41 which derives signals fromthe output voltage across the load; current logic" 42 which derivessignals through a sensing means or current transformer 45 from theoutput current, each of the said signals being combined by a so-calledcomparator as indicated by box representation 46. The comparatorsfunction is to accept the permissive (designed as P) or inhibit(designated as In) signal from the above-mentioned three logics and todeliver a proper signal to the gating drive circuit" 43 to operate anappropriate gate 51-56 of the selected thyristor l0, ll, 12, 16, 17, or18.

Maximum output power is realized when switch connected to phase A iscaused to conduct during the time period when A is the most positive ofthe input phases; when switch 11 connected to phase B is caused toconduct during the time period when B is the most positive, when switch12 connected to phase C is caused to conduct during the period whenphase C is most positive; and similarly when thyristors 16, 17 and 18with their cathodes connected to phases A, B, and C, respectively, arecaused to conduct during the time periods when the phases A, B, and Care most negative.

The time periods during which any one of the thyristors may conduct aretermed envelopes. The envelope portion of the control 40 takes as inputinformation the instantaneous value of the input voltages of phases A,B, and C and the conditions for maximum power are shown in FIG. 3. Inorder that a balanced three-phase load be presented to the supply, it isessential that these envelopes be of substantially equal duration.

Power control can be achieved as for example by shifting the envelopewith respect to the input voltage, as shown in FIG. 8 as hereinafterdescribed.

As shown in FIG. 4, a second portion of the control circuit 41 takes asits input information the instantaneous value of the output voltageacross the load L and based on this information directs the firingpulses to the switches so that positive switches 10, 11, and 12 andnegative switches 16, 17, 18 are supplied with firing pulses to sustainthe oscillation in the tuned load circuit and respond to changes in theload resonant frequency. During normal steady state conditions of thisconverter, a fixed time interval is maintained between the voltage zerocross-over instant and the initiation of firing. This time interval maybe positive or negative and is relatively small with respect to theperiod of the output voltage.

The switches utilized in this equipment are of the type that require adefinite time to change from their blocking or non-conducting state totheir conducting state; and then after cessation of current a definitebut different period of time to regain their blocking ability.

The current logic portion of the control 42 takes as its input a signalwhich is proportional to the output current L, from the currenttransformer 45 and permits the delivery of an output pulse based on thevoltage zero cross-over instant to be delivered to the proper thyristorso that said output pulse is permitted as soon after the cessation ofthe preceding output current pulse following a safe period for thedevice to regain itsblocking ability, as indicated by t FIG. 5illustrates this effect.

Thus in normal continuous operation of the converter, the pulses aredirected to the gate of the switch connected to the appropriate inputphase of the proper polarity at the normal time with respect to theoutput voltage zero cross-over instant or as soon thereafter aspermitted by the safe period provided following the cessation of thepreceding current pulse.

If a transient occurs and for any reason the time period of the outputcurrent is extended with respect to the output voltage half periodjeopardizing the continued safe operation of the converter, thecomparator portion 46 of the control automatically delays the pulseuntil a safe time interval has elapsed. The delaying action of the pulsecontinues until normal steady state operation is resumed.

To initiate the operation of the converter, a single pulse is injectedinto the voltage logic circuit 40 at terminal 71. This causes a firingpulse to be delivered to the appropriate switch and the first outputcurrent pulse flows. Subsequent voltage pulses are generated by thevoltage logic based on the output voltage zero cross-over instant and inthis manner the converter is able to follow changes in the resonantfrequency of the load circuit.

Under certain load conditions, we have found it desirable to avoidtransients and to assure smooth startup to control the timing of theinitial starting pulse with respect to the input voltage wave andvarious means for achieving said initial pulse may be employed as by aswitch which when in closed position takes a starting pulse from theenvelope logic 40. We have found a preferred setting for said initialstarting pulse is within 0 to 60 range in the first half cycle of saidwave.

It is often desirable to adjust the power delivered to the load, i.e.after a melt has been completed, the power is reduced to a holding leveluntil the metal is poured off. This can be accomplished in discretesteps by adjusting the ratio of the output transformer 38, as by taps39, etc. It is likewise desirable to make a smooth and continuousadjustment of the output power, i.e. to maintain a specific metaltemperature. This can be accomplished by adjustment of the phase or timerelationship between the envelope and supply voltage. As shown in FIG.8, the envelope has been timed to provide maximumpower and the outputcurrent and the portion of that current flowing in phase A asillustrated. As the envelope is shifted, the effective value of voltagedelivered to thyristors and 16 is 'reduced and the output power willsimilarly be reduced. FIG. 9 illustrates this effect. Obviously, inorder to maintain a balance between the three-phase supply lines, theenvelopes must be shifted an equal amount.

We have discovered that smooth transient free operation of our converteris considerably enhanced by shifting the envelope forward as shown inFIG. 9 with respect to the input voltage wave. A typical curve of therelationship between power and the forward shift from the maximum powerpoint is shown in FIG. 6a.

It is possible to regulate the power at a fixed point other than maximumavailable by sensing the load voltage on the secondary side of thetransformer 38 and obtaining a current signal from a current transformer61, comparing the said output power to a desired operating level,indicated by an operator adjustable potentiometer as shown at 60, andsignalling by means 62 connected to envelope logic 40 by a line 63 toadjust the phase of 64-69 of the envelope logic 40 in accordance withthe curve shown in FIG. 6a to maintain constant power output within thelimits described.

The relationship between phase shift and output current and phase shiftand output voltage are shown in FIGS. 6b and 6c. The control can beadapted to provide limit protection for load voltage and output current.A signal proportional to the load voltage can be used as an input to avoltage limit circuit, not herein illustrated. If the RMS voltagedelivered to the load exceeds a safe predetermined limit, the envelopeis shifted to reduce the load voltage as shown in FIG. 6c. Similarly, asignal proportional to the output current can be used as an input to acurrent limit circuit, not herein illustrated. If the RMS currentdelivered to the load exceeds a safe predetermined limit, the envelopeis shifted to reduce the output current as shown in FIG. 6b.

Under dynamic operating conditions, as stated hereinbefore, varioustransient conditions caused by sudden load change, change in theintended power level, line voltage surges, it is essential to adjust theoperating frequency, the timing of the pulses with respect to the outputvoltage, and the envelope with respect to the incoming voltage inresponse to changes in the load or process requirements. Thesecan bestbe illustrated by assuming the converter to be operating satisfactorilyat maximum power with adequate thyristor turn-off time. The outputs ofthe logic and the thyristor gate drive circuit for this condition areillustrated in FIG. 7a.

As the resonant frequency of the load changes, the feedback from theload through the voltage logic products output pulses at the appropriatefrequency. If any transient described in the hereinbefore describedcircuit result in insufficient turnoff time, the pulses are delayed bythe comparison circuitry until a safe time has elapsed, see FIG. 7b.

It will be understood that many changes in the details of the inventionas herein described and illustrated may be made without, however,departing from the spirit thereof or the scope of the appended claims.

What I claim is:

1. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a substantially fixed timerelationship between initiating of firing pulses for the said solidstate switching devices during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse.

2. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a fixed time relationshipbetween initiation of each firing pulse for said solid state switchingdevices and the corresponding output voltage zero cross-over instantduring steady state conditions and to automatically modify said timerelationship under transient conditions which results in substantiallengthening of an output current pulse, so as to obtain a safe intervalbetween the cessation of said lengthened output current pulse and theinitiation of the subsequent firing pulse.

3. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output I current, and means for combining signalsderived from said sensing means to provide initiation of firing pulsesfor said solid state switching devices substantially at each outputvoltage zero cross-over instant during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse, soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse.

4. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a substantially fixed timerelationship between initiating of firing pulses for the said solidstate switching devices during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,wherein said control means include sensing means for sensing theinstantaneous value of the input voltage, means for initiating astarting pulse directing said pulse to an appropriate solid stateswitching device at a predetermined phase angle with respect to line toneutral voltage zero-over instant,

5. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a fixed time relationshipbetween initiation of each firing pulse for said solid state switchingdevices and the corresponding output voltage zero cross-over instantduring steady state conditions and to automatically modify said timerelationship under transient conditions which results in substantiallengthening of an output current pulse, so as to obtain a safe intervalbetween the cessation of said lengthened output current pulse and theinitiation of the subsequent firing pulse, wherein said control meansinclude sensing means for sensing the instantaneous value of the inputvoltage, means for initiating a starting pulse directing said pulse toan appropriate solid state switching device at a predetermined phaseangle with respect to line to neutral voltage zero cross-over instant.

6. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide initiation of firing pulsesfor said solid state switching devices substantially at each outputvoltage zero cross-over instant during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse, soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,wherein said control means include sensing means for sensing theinstantaneous value of the input voltage, means for initiating astarting pulse directing said pulse to an appropriate solid stateswitching device at a predetermined phase angle with respect to line toneutral voltage cross-over instant.

7. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a substantially fixed timerelationship between initiating of firing pulses for the said solidstate switching devices during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,said control means including sensing means for sensing the instantaneousvalue of the input voltage, means for initiating a starting pulsedirecting said pulse to an appropriate solid state switching device at apredetermined phase angle with respect to line to neutral voltage zerocross-over instant and said control means initiating said starting pulseinitiate said pulse within a 0 to 60 range in the first half cycle ofthe input voltage wave.

8. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a substantially fixed timerelationship between initiation of each firing pulse for said solidstate switching devices and the corresponding output voltage zerocross-over instant during steady state conditions and to automaticallymodify said time relationship under transient conditions which resultsin substantial lengthening of an output current pulse, so as to obtain asafe interval between the cessation of said lengthened output currentpulse and the initiation of the subsequent firing pulse, said controlmeans including sensing means for sensing the instantaneous value of theinput voltage, means for initiating a starting pulse directing saidpulse to an appropriate solid state switching device at a predeterminedphase angle with respect to line to neutral voltage zero cross-overinstant and said control means initiating said starting pulse initiatesaid pulse within a 0 to 60 range in the first half cycle of the inputvoltage wave.

9. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide initiation of firing pulsesfor said solid state switching devices substantially at each outputvoltage zero cross-over instant during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse, soas to obtain a safe interval between the cessation of said lengthenedoutputcurrent pulse and the initiation of the subsequent firing pulse,said control means including sensing means for sensing the instantaneousvalue of the input voltage, means for initiating a starting pulsedirecting said pulse to an appropriate solid state switching device at apredetermined phase angle with respect to line to neutral voltage zerocrossover instant and said control means initiating said starting pulseinitiate said pulse within a to 60 range in the first half cycle of theinput voltage wave.

10. Control means for an AC to AC frequency converter as claimed inclaim 1, wherein said control means include sensing means for sensingthe instantaneous value of the line to neutral supply voltage andproducing substantially equal envelopes controlling the time periodduring which each solid state switching device conducts output current,and means whereby the time relationship of said envelopes to said lineto neutral supply voltage is shifted to provide control of output power.

11. Control means for an AC to AC frequency converter as claimed inclaim 2, wherein said control means include sensing means for sensingthe instantaneous value of the line to neutral supply voltage andproducing substantially equal envelopes controlling the time periodduring which each solid state switching device conducts output current,and means whereby the time relationship of said envelopes to said lineto neutral supply voltage is shifted to provide control of output power.

12. Control means for an AC to AC frequency converter as claimed inclaim 3, wherein said control means include sensing means for sensingthe instantaneous value of the line to neutral supply voltage andproducing substantially equal envelopes controlling the time periodduring which each solid state switching device conducts output current,and means whereby the time relationship of said envelopes to said lineto neutral supply voltage is shifted to provide control of output power.

13. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a substantially fixed timerelationship between initiating of firing pulses for the said solidstate switching devices during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse soas to obtain a safe interval between the cessation of the subsequentfiring pulse, wherein said control means also include sensing means forsensing the instantaneous value of the line to neutral supply voltageand producing substantially equal envelopes controlling the time periodduring which each solid state switching device conducts, and meanswhereby the time relationship of said envelopes and said line to neutralsupply voltage is modifiedby shifting said envelopes ahead in time toreduce output power.

14. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a fixed time relationshipbetween initiation of each firing pulse for said solid state switchingdevices and the corresponding output voltage zero cross-over instantduring steady state conditions and to automatically modify said timerelationship under transient conditions which results in substantiallengthening of an output current pulse, so as to obtain a safe intervalbetween the cessation of said lengthened output current pulse and theinitiation of the subsequent firing pulse, wherein said control meansalso include sensing means for sensing the instantaneous value of theline to neutral supply voltage and producing substantially equalenvelopes controlling the time period during which each solid stateswitching device conducts, and means whereby the time relationship ofsaid envelopes and said line to neutral supply voltage is modified byshifting said envelopes ahead in time to reduce output power.

15. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide initiation of firing pulsesfor said solid state switching devices substantially at each outputvoltage zero cross-over instant during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse, soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,wherein said control means also include sensing means for sensing theinstantaneous value of the line to neutral supply voltage and producingsubstantially equal envelopes controlling the time period during whicheach solid state switching device conducts, and means whereby the timerelationship of said envelopes and said line to neutral supply voltageis modified by shifting said envelopes ahead in time to reduce outputpower.

16. Control means for an AC to AC frequency converter as claimed inclaim 13, wherein the power is regulated by sensing of the outputvoltage and output current and adjusting the phase of the envelopelogic.

17. Control means for an AC to AC frequency converter as claimed inclaim 14, wherein the power is regulated by sensing of the outputvoltage and output current and adjusting the phase of the envelopelogic.

18. Control means for an AC to AC frequency converter as claimed inclaim 15, wherein the power is regulated by sensing of the outputvoltage and output current and adjusting the phase of the envelopelogic.

19. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected all to a polyphase lower frequencysource to provide single phase power at a higher frequency to a loadcomprising an induction heating coil and parallel capacitance, saidcontrol means including sensing means for sensing the instantaneousvalues of the output voltage and output current, and means for combiningsignals derived from said sensing means to provide a substantially fixedtime relationship between initiating of firing pulses for the said solidstate switching devices during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,said sensing means are provided to sense instantaneous line to neutralsupply voltage and power control is obtained by shifting the timerelationship between the envelope and the line to neutral supply voltageand the envelope is shifted forward with respect to the input voltagewave, wherein said control includes a signal proportional to the outputvoltage providing an input to a voltage limit circuit which shifts theenvelope forward with respect to the line to neutral supply voltage.

20. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a fixed time relationshipbetween initiation of each firing pulse for said solid state switchingdevices and the corresponding output voltage zero cross-over instantduring steady state conditions and to automatically modify said timerelationship under transient conditions which results in substantiallengthening of an output current pulse, so as to obtain a safe intervalbetween the cessation of said lengthened output current pulse and theinitiation of the subsequent firing pulse, said sensing means areprovided to sense instantaneous line to neutral supply voltage and powercontrol is obtained by shifting the time relationship between theenvelope and the line to neutral supply voltage and the envelope isshifted forward with respect to the input voltage wave, wherein saidcontrol includes a signal proportional to the output voltage providingan input to a voltage limit circuit which shifts the envelope forwardwith respect to the line to neutral supply voltage.

21. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide initiation of firing pulsesfor said solid state switching devices substantially at each outputvoltage zero cross-over instant during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse, soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the in itiation of the subsequent firing pulse,said sensing means are provided to sense instantaneous line to neutralsupply voltage and power control is obtained by shifting the timerelationship between the envelope and the line to neutral supply voltageand the envelope is shifted forward with respect to the input voltagewave, wherein said control includes a signal proportional to the outputvoltage providing an input to a voltage limit circuit which shifts theenvelope forward with respect to the line to neutral supply voltage.

22. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a substantially fixed timerelationship between initiating of firing pulses for the solid stateswitching devices during steady state conditions and to automaticallymodify said time relationship under transient conditions which resultsin substantial lengthening of an output current pulse so as to obtain asafe interval between the cessation of said lengthened output currentpulse and the initiation of the subsequent firing pulse, said sensingmeans are provided to sense instantaneous line to neutral supply voltageand power control is obtained by shifting the time relation ship betweenthe envelope and the line to neutral supply voltage and the envelope isshifted forward with respect to the input voltage wave, wherein saidcontrol means includes a signal proportional to the output current andprovides an input to a current limit circuit which shifts the envelopeforward with respect to the line to neutral supply voltage.

23. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected toa polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a fixed time relationshipbetween initiation of each firing pulse for said solid state switchingdevices and the corresponding output voltage zero cross-over instantduring steady state conditions and to automatically modify said timerelationship under transient conditions which results in substantiallengthening of an output current pulse, so as to obtain a safe intervalbetween the cessation of said lengthened output current pulse and theinitiation of the subsequent firing pulse, said sensing means areprovided to sense instantaneous line to neutral supply voltage and powercontrol is obtained by shifting the time relationship between theenvelope and the line to neutral supply voltage and the envelope isshifted forward with respect to the input voltage wave, wherein saidcontrol means includes a signal proportional to the output current andprovides an input to a current limit circuit which shifts the envelopeforward with respect to the line to neutral supply voltage.

24. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide initiation of firing pulsesfor said solid state switching devices substantially at each outputvoltage zero cross-over instant during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse, soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,said sensing means are provided to sense instantaneous line to neutralsupply voltage and power control is obtained by shifting the timerelationship between the envelope and the line to neutral supply voltageand the envelope is shifted forward with respect to the input voltagewave, wherein said control means includes a signal proportional to theoutput current and provides an input to a current limit circuit whichshifts the envelope forward with respect to the line to neutral supplyvoltage.

1. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a substantially fixed timerelationship between initiating of firing pulses for the said solidstate switching devices during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse.2. Control means for an AC to AC frequency converter utilizing solidstate switching devices connected to a polyphase lower frequency sourceto provide single phase power at a higher frequency to a load comprisingan induction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsdErived from said sensing means to provide a fixed time relationshipbetween initiation of each firing pulse for said solid state switchingdevices and the corresponding output voltage zero cross-over instantduring steady state conditions and to automatically modify said timerelationship under transient conditions which results in substantiallengthening of an output current pulse, so as to obtain a safe intervalbetween the cessation of said lengthened output current pulse and theinitiation of the subsequent firing pulse.
 3. Control means for an AC toAC frequency converter utilizing solid state switching devices connectedto a polyphase lower frequency source to provide single phase power at ahigher frequency to a load comprising an induction heating coil andparallel capacitance, said control means including sensing means forsensing the instantaneous values of the output voltage and outputcurrent, and means for combining signals derived from said sensing meansto provide initiation of firing pulses for said solid state switchingdevices substantially at each output voltage zero cross-over instantduring steady state conditions and to automatically modify said timerelationship under transient conditions which results in substantiallengthening of an output current pulse, so as to obtain a safe intervalbetween the cessation of said lengthened output current pulse and theinitiation of the subsequent firing pulse.
 4. Control means for an AC toAC frequency converter utilizing solid state switching devices connectedto a polyphase lower frequency source to provide single phase power at ahigher frequency to a load comprising an induction heating coil andparallel capacitance, said control means including sensing means forsensing the instantaneous values of the output voltage and outputcurrent, and means for combining signals derived from said sensing meansto provide a substantially fixed time relationship between initiating offiring pulses for the said solid state switching devices during steadystate conditions and to automatically modify said time relationshipunder transient conditions which results in substantial lengthening ofan output current pulse so as to obtain a safe interval between thecessation of said lengthened output current pulse and the initiation ofthe subsequent firing pulse, wherein said control means include sensingmeans for sensing the instantaneous value of the input voltage, meansfor initiating a starting pulse directing said pulse to an appropriatesolid state switching device at a predetermined phase angle with respectto line to neutral voltage zero-over instant.
 5. Control means for an ACto AC frequency converter utilizing solid state switching devicesconnected to a polyphase lower frequency source to provide single phasepower at a higher frequency to a load comprising an induction heatingcoil and parallel capacitance, said control means including sensingmeans for sensing the instantaneous values of the output voltage andoutput current, and means for combining signals derived from saidsensing means to provide a fixed time relationship between initiation ofeach firing pulse for said solid state switching devices and thecorresponding output voltage zero cross-over instant during steady stateconditions and to automatically modify said time relationship undertransient conditions which results in substantial lengthening of anoutput current pulse, so as to obtain a safe interval between thecessation of said lengthened output current pulse and the initiation ofthe subsequent firing pulse, wherein said control means include sensingmeans for sensing the instantaneous value of the input voltage, meansfor initiating a starting pulse directing said pulse to an appropriatesolid state switching device at a predetermined phase angle with respectto line to neutral voltage zero cross-over instant.
 6. Control means foran AC to AC frequency converter utilizing solid state switching devicesconnected to a polyphaSe lower frequency source to provide single phasepower at a higher frequency to a load comprising an induction heatingcoil and parallel capacitance, said control means including sensingmeans for sensing the instantaneous values of the output voltage andoutput current, and means for combining signals derived from saidsensing means to provide initiation of firing pulses for said solidstate switching devices substantially at each output voltage zerocross-over instant during steady state conditions and to automaticallymodify said time relationship under transient conditions which resultsin substantial lengthening of an output current pulse, so as to obtain asafe interval between the cessation of said lengthened output currentpulse and the initiation of the subsequent firing pulse, wherein saidcontrol means include sensing means for sensing the instantaneous valueof the input voltage, means for initiating a starting pulse directingsaid pulse to an appropriate solid state switching device at apredetermined phase angle with respect to line to neutral voltagecross-over instant.
 7. Control means for an AC to AC frequency converterutilizing solid state switching devices connected to a polyphase lowerfrequency source to provide single phase power at a higher frequency toa load comprising an induction heating coil and parallel capacitance,said control means including sensing means for sensing the instantaneousvalues of the output voltage and output current, and means for combiningsignals derived from said sensing means to provide a substantially fixedtime relationship between initiating of firing pulses for the said solidstate switching devices during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,said control means including sensing means for sensing the instantaneousvalue of the input voltage, means for initiating a starting pulsedirecting said pulse to an appropriate solid state switching device at apredetermined phase angle with respect to line to neutral voltage zerocross-over instant and said control means initiating said starting pulseinitiate said pulse within a 0* to 60* range in the first half cycle ofthe input voltage wave.
 8. Control means for an AC to AC frequencyconverter utilizing solid state switching devices connected to apolyphase lower frequency source to provide single phase power at ahigher frequency to a load comprising an induction heating coil andparallel capacitance, said control means including sensing means forsensing the instantaneous values of the output voltage and outputcurrent, and means for combining signals derived from said sensing meansto provide a substantially fixed time relationship between initiation ofeach firing pulse for said solid state switching devices and thecorresponding output voltage zero cross-over instant during steady stateconditions and to automatically modify said time relationship undertransient conditions which results in substantial lengthening of anoutput current pulse, so as to obtain a safe interval between thecessation of said lengthened output current pulse and the initiation ofthe subsequent firing pulse, said control means including sensing meansfor sensing the instantaneous value of the input voltage, means forinitiating a starting pulse directing said pulse to an appropriate solidstate switching device at a predetermined phase angle with respect toline to neutral voltage zero cross-over instant and said control meansinitiating said starting pulse initiate said pulse within a 0* to 60*range in the first half cycle of the input voltage wave.
 9. Controlmeans for an AC to AC frequency converter utilizing solid stateswitching devices connected to a polyphase lower frequency source toprovide single phase power at a higher frequency to a load comprising aninduction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide initiation of firing pulsesfor said solid state switching devices substantially at each outputvoltage zero cross-over instant during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse, soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,said control means including sensing means for sensing the instantaneousvalue of the input voltage, means for initiating a starting pulsedirecting said pulse to an appropriate solid state switching device at apredetermined phase angle with respect to line to neutral voltage zerocross-over instant and said control means initiating said starting pulseinitiate said pulse within a 0* to 60* range in the first half cycle ofthe input voltage wave.
 10. Control means for an AC to AC frequencyconverter as claimed in claim 1, wherein said control means includesensing means for sensing the instantaneous value of the line to neutralsupply voltage and producing substantially equal envelopes controllingthe time period during which each solid state switching device conductsoutput current, and means whereby the time relationship of saidenvelopes to said line to neutral supply voltage is shifted to providecontrol of output power.
 11. Control means for an AC to AC frequencyconverter as claimed in claim 2, wherein said control means includesensing means for sensing the instantaneous value of the line to neutralsupply voltage and producing substantially equal envelopes controllingthe time period during which each solid state switching device conductsoutput current, and means whereby the time relationship of saidenvelopes to said line to neutral supply voltage is shifted to providecontrol of output power.
 12. Control means for an AC to AC frequencyconverter as claimed in claim 3, wherein said control means includesensing means for sensing the instantaneous value of the line to neutralsupply voltage and producing substantially equal envelopes controllingthe time period during which each solid state switching device conductsoutput current, and means whereby the time relationship of saidenvelopes to said line to neutral supply voltage is shifted to providecontrol of output power.
 13. Control means for an AC to AC frequencyconverter utilizing solid state switching devices connected to apolyphase lower frequency source to provide single phase power at ahigher frequency to a load comprising an induction heating coil andparallel capacitance, said control means including sensing means forsensing the instantaneous values of the output voltage and outputcurrent, and means for combining signals derived from said sensing meansto provide a substantially fixed time relationship between initiating offiring pulses for the said solid state switching devices during steadystate conditions and to automatically modify said time relationshipunder transient conditions which results in substantial lengthening ofan output current pulse so as to obtain a safe interval between thecessation of the subsequent firing pulse, wherein said control meansalso include sensing means for sensing the instantaneous value of theline to neutral supply voltage and producing substantially equalenvelopes controlling the time period during which each solid stateswitching device conducts, and means whereby the time relationship ofsaid envelopes and said line to neutral supply voltage is modified byshifting said envelopes ahead in time to reduce output power. 14.Control means for an AC to AC frequency converter utilizing solid stateswitching devices connected to a polyphase lower frequency source toprovide single phase power at a higher frequency to a load comprising aninduction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a fixed time relationshipbetween initiation of each firing pulse for said solid state switchingdevices and the corresponding output voltage zero cross-over instantduring steady state conditions and to automatically modify said timerelationship under transient conditions which results in substantiallengthening of an output current pulse, so as to obtain a safe intervalbetween the cessation of said lengthened output current pulse and theinitiation of the subsequent firing pulse, wherein said control meansalso include sensing means for sensing the instantaneous value of theline to neutral supply voltage and producing substantially equalenvelopes controlling the time period during which each solid stateswitching device conducts, and means whereby the time relationship ofsaid envelopes and said line to neutral supply voltage is modified byshifting said envelopes ahead in time to reduce output power. 15.Control means for an AC to AC frequency converter utilizing solid stateswitching devices connected to a polyphase lower frequency source toprovide single phase power at a higher frequency to a load comprising aninduction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide initiation of firing pulsesfor said solid state switching devices substantially at each outputvoltage zero cross-over instant during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse, soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,wherein said control means also include sensing means for sensing theinstantaneous value of the line to neutral supply voltage and producingsubstantially equal envelopes controlling the time period during whicheach solid state switching device conducts, and means whereby the timerelationship of said envelopes and said line to neutral supply voltageis modified by shifting said envelopes ahead in time to reduce outputpower.
 16. Control means for an AC to AC frequency converter as claimedin claim 13, wherein the power is regulated by sensing of the outputvoltage and output current and adjusting the phase of the envelopelogic.
 17. Control means for an AC to AC frequency converter as claimedin claim 14, wherein the power is regulated by sensing of the outputvoltage and output current and adjusting the phase of the envelopelogic.
 18. Control means for an AC to AC frequency converter as claimedin claim 15, wherein the power is regulated by sensing of the outputvoltage and output current and adjusting the phase of the envelopelogic.
 19. Control means for an AC to AC frequency converter utilizingsolid state switching devices connected to a polyphase lower frequencysource to provide single phase power at a higher frequency to a loadcomprising an induction heating coil and parallel capacitance, saidcontrol means including sensing means for sensing the instantaneousvalues of the output voltage and output current, and means for combiningsignals derived from said sensing means to provide a substantially fixedtime relationship between initiating of firing pulses for the said solidstate switching devices during steady state conditions and toautomatically modify said tiMe relationship under transient conditionswhich results in substantial lengthening of an output current pulse soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,said sensing means are provided to sense instantaneous line to neutralsupply voltage and power control is obtained by shifting the timerelationship between the envelope and the line to neutral supply voltageand the envelope is shifted forward with respect to the input voltagewave, wherein said control includes a signal proportional to the outputvoltage providing an input to a voltage limit circuit which shifts theenvelope forward with respect to the line to neutral supply voltage. 20.Control means for an AC to AC frequency converter utilizing solid stateswitching devices connected to a polyphase lower frequency source toprovide single phase power at a higher frequency to a load comprising aninduction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a fixed time relationshipbetween initiation of each firing pulse for said solid state switchingdevices and the corresponding output voltage zero cross-over instantduring steady state conditions and to automatically modify said timerelationship under transient conditions which results in substantiallengthening of an output current pulse, so as to obtain a safe intervalbetween the cessation of said lengthened output current pulse and theinitiation of the subsequent firing pulse, said sensing means areprovided to sense instantaneous line to neutral supply voltage and powercontrol is obtained by shifting the time relationship between theenvelope and the line to neutral supply voltage and the envelope isshifted forward with respect to the input voltage wave, wherein saidcontrol includes a signal proportional to the output voltage providingan input to a voltage limit circuit which shifts the envelope forwardwith respect to the line to neutral supply voltage.
 21. Control meansfor an AC to AC frequency converter utilizing solid state switchingdevices connected to a polyphase lower frequency source to providesingle phase power at a higher frequency to a load comprising aninduction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide initiation of firing pulsesfor said solid state switching devices substantially at each outputvoltage zero cross-over instant during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse, soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,said sensing means are provided to sense instantaneous line to neutralsupply voltage and power control is obtained by shifting the timerelationship between the envelope and the line to neutral supply voltageand the envelope is shifted forward with respect to the input voltagewave, wherein said control includes a signal proportional to the outputvoltage providing an input to a voltage limit circuit which shifts theenvelope forward with respect to the line to neutral supply voltage. 22.Control means for an AC to AC frequency converter utilizing solid stateswitching devices connected to a polyphase lower frequency source toprovide single phase power at a higher frequency to a load comprising aninduction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from sAid sensing means to provide a substantially fixed timerelationship between initiating of firing pulses for the solid stateswitching devices during steady state conditions and to automaticallymodify said time relationship under transient conditions which resultsin substantial lengthening of an output current pulse so as to obtain asafe interval between the cessation of said lengthened output currentpulse and the initiation of the subsequent firing pulse, said sensingmeans are provided to sense instantaneous line to neutral supply voltageand power control is obtained by shifting the time relationship betweenthe envelope and the line to neutral supply voltage and the envelope isshifted forward with respect to the input voltage wave, wherein saidcontrol means includes a signal proportional to the output current andprovides an input to a current limit circuit which shifts the envelopeforward with respect to the line to neutral supply voltage.
 23. Controlmeans for an AC to AC frequency converter utilizing solid stateswitching devices connected to a polyphase lower frequency source toprovide single phase power at a higher frequency to a load comprising aninduction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide a fixed time relationshipbetween initiation of each firing pulse for said solid state switchingdevices and the corresponding output voltage zero cross-over instantduring steady state conditions and to automatically modify said timerelationship under transient conditions which results in substantiallengthening of an output current pulse, so as to obtain a safe intervalbetween the cessation of said lengthened output current pulse and theinitiation of the subsequent firing pulse, said sensing means areprovided to sense instantaneous line to neutral supply voltage and powercontrol is obtained by shifting the time relationship between theenvelope and the line to neutral supply voltage and the envelope isshifted forward with respect to the input voltage wave, wherein saidcontrol means includes a signal proportional to the output current andprovides an input to a current limit circuit which shifts the envelopeforward with respect to the line to neutral supply voltage.
 24. Controlmeans for an AC to AC frequency converter utilizing solid stateswitching devices connected to a polyphase lower frequency source toprovide single phase power at a higher frequency to a load comprising aninduction heating coil and parallel capacitance, said control meansincluding sensing means for sensing the instantaneous values of theoutput voltage and output current, and means for combining signalsderived from said sensing means to provide initiation of firing pulsesfor said solid state switching devices substantially at each outputvoltage zero cross-over instant during steady state conditions and toautomatically modify said time relationship under transient conditionswhich results in substantial lengthening of an output current pulse, soas to obtain a safe interval between the cessation of said lengthenedoutput current pulse and the initiation of the subsequent firing pulse,said sensing means are provided to sense instantaneous line to neutralsupply voltage and power control is obtained by shifting the timerelationship between the envelope and the line to neutral supply voltageand the envelope is shifted forward with respect to the input voltagewave, wherein said control means includes a signal proportional to theoutput current and provides an input to a current limit circuit whichshifts the envelope forward with respect to the line to neutral supplyvoltage.